Integrated circuit fabrication processes that use reactive plasmas are commonplace in today's semiconductor production lines. The term reactive plasma is meant to describe a discharge in which ionization and fragmentation of gases take place and produce chemically active species, frequently oxidizing and/or reducing agents. Such plasmas are reactive both in the gas phase and with solid surfaces exposed to them. When reactive plasmas were first used in semiconductor factories, in the early 1970's, little was known about the chemistry and physics of these discharges. Today, there is a broad base of empirical knowledge, some qualitative understanding of active mechanisms, and even some detailed understanding of specific isolated phenomena. This insight now allows for the tailoring of plasma processes to meet the more stringent requirements of today's submicron device features.
When using plasma processing, many problems can occur which will limit yields or reduce manufacturing throughput. For example, water vapor which enters the plasma processing chamber can detrimentally affect the production of the semiconductor wafers. This problem must be detected quickly or there is a great risk of poor yields.
Another problem with processing is the build up of contaminants on the chamber walls. Presently, typical processing factories require the chamber walls to be periodically cleaned after given time periods, for example, after a certain number of wafers have been processed. This technique presents some major drawbacks if the rate of buildup is not known, which it is usually not. For example, if the chamber walls are not cleaned often enough, there is a risk that some of the wafers which have been processed in the later stages will be contaminated and will suffer from poor yields. On the other hand, if the chamber is cleaned too often, then this represents a reduction in throughput will result. Therefore, there is currently a need for an in-situ real time detector of chamber wall cleanliness.
Accordingly, improvements which overcome any or all of the problems are presently desirable.